It is proposed to construct a series of hybrid microorganisms by combining segments of the genomes of two or more organisms into one single microorganism chromosome. Previous work in this area has involved bringing one or several genes into higher cells, such as the construction of a transgenic mouse, or else bringing one or several genes into a microorganism via a plasmid vector that remains in the cytoplasm. It is proposed here, however, to create new microorganisms by introducing large (greater than 10 genes, and as many as 100 to 1,000 genes) genetic segments into the cell and incorporating them directly into the chromosome by genetic engineering. In many cases the constructed hybrids will have a series of desired properties derived from different microorganisms, such as nitrogen fixation pathways, oil slick degradation pathways, and antibiotic synthesis pathways. These new species of microorganisms can be constructed in either a planned, directed manner, or by random combination of segments of different genomes to open up an era of "combinatorial genomes."